CONDENSED CONSOLIDATED INCOME STATEMENTS (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Operating revenues	$ 2,304.5	$ 2,194.8
Operating expenses
Cost of sales	941.1	838.9
Other operation and maintenance	501.9	531.5
Depreciation and amortization	194.6	187.9
Property and revenue taxes	49.6	47.2
Total operating expenses	1,687.2	1,605.5
Operating income	617.3	589.3
Equity in earnings of transmission affiliate	41.9	38.5
Other income, net	15.7	32.7
Interest expense	104.7	100.9
Other expense	(47.1)	(29.7)
Income before income taxes	570.2	559.6
Income tax expense	213.3	213.1
Net income	356.9	346.5
Preferred stock dividends of subsidiary	0.3	0.3
Net income attributed to common shareholders	$ 356.6	$ 346.2
Earnings per share
Basic (in dollars per share)	$ 1.13	$ 1.10
Diluted (in dollars per share)	$ 1.12	$ 1.09
Weighted average common shares outstanding
Basic (in shares)	315.6	315.7
Diluted (in shares)	317.2	317.1
Dividends per share of common stock (in dollars per share)	$ 0.5200	$ 0.4950